Mechanistic Model of Polypropylene and Polystyrene Solid Fuel Combustion for Supersonic Propulsion

This paper presents a novel mechanistic model for the thermal decomposition and combustion of solid polymer fuels, specifically polypropylene and polystyrene, developed using the method of moments. The model captures the complex multiphase interactions within the thin foam-like decomposition layer at the polymer surface, coupling volatile production in the condensed phase with gas-phase oxidation through a foam-layer formulation. Key polymer pyrolysis reactions, including chain fission, radical recombination, hydrogen abstraction, and $\beta$-scission, are represented in a reduced-order moment framework, enabling efficient yet detailed simulation of molecular weight evolution and volatile species formation. Gas-phase combustion of volatiles is modeled using a detailed styrene and propylene oxidation mechanism validated against shock tube ignition delay data. Two momentum coupling models within the foam layer are explored, with the bubble-driven flow model showing improved agreement with experimental regression rates and flame stand-off distances measured in counterflow diffusion flames. The coupled pyrolysis-combustion model demonstrates excellent agreement with pyrolysis experiments and significantly enhanced predictive capability for burn rates and flame structure compared to prior global models. Sensitivity analyses highlight the influence of foam parameters on combustion performance. This integrated framework advances the understanding and predictive modeling of solid polymer fuel combustion for supersonic propulsion applications.